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Aviation History
1928
1928 - 0668.PDF
56 TO JDLY 19, 1928FLIGHT THE AIRCRAFT ENGINEER this is so, but the successful forming of very soft materials is often far more difficult than corrugating steel of S. 40 standard ; for example, in one case, four pairs of rolls were required to produce a certain section from material of proof stress 18 tons per sq. in., and only two pairs when the proof stress was 70 tons per sq. in., the gauges being the same in each ease, but that was simply because the soft material could not withstand the work of deformation on two pairs of rolls without buckling and it had to be done by more gradual stages. It is clear from the foregoing considerations that this matter of corrugating and section forming is a subject that teems with interest in both its theoretical and practical aspects. FACTORS IN SEAPLANE FLOAT DESIGN By WM. MTJKRO Conflicting requirements in the design of seaplane floats necessitate a compromise in shape to give the most satis- factory all-round results. These requirements may be stated in order of importance as : (1) Clean water performance and stability. (2) Reasonable air resistance. It is possible to eliminate a considerable amount of experi- mental work, and to draft out speedily a good lines drawing, with the aid of a suitable set of coefficients compiled from other successful floats, for a variety of machines. The coefficients used in this method are (a) Midship area coefficient. (b) Coefficient of fineness of water-plane. (c) Block coefficient. (d) Linear ratios. (e) Moment of inertia coefficient. (a) Is the ratio of the area of the midship section of the float to the rectangle ABCD which encloses it. (See Fig. 1.) (b) Is the ratio of the water plane area of the float to the rectangle EFGH which encloses it. (See Fig. 2.) (c) Is the ratio of the volume of the float to the volume of a block having the same overall length, same extreme breadth, and same extreme depth. This will give the proportions of a normal British float of accepted present-day design for a seaplane inside the weight range of 3,000-7,000 lbs., the flotation system being twin- floats with turtle deck and vee bottom, and with one step approximately at midships. The turtle deck is the favourite type in England because of (a) the strength of the section, and (b) the immediate drainage of water from the float top. No great difficulty has been experienced in providing adequate walkways along the float top where necessary. The step should be situated close to the centre of gravity of the machine, since, when getting off, the machine rides on the step. The function of the step is to create a break in the con- tinuity of flow of the water, which reduces suction and helps the machine to get off the water. The vee shape of planing bottom has the obvious advant- age of reducing shock when alighting, and is adopted for this reason, although inferior to the flat planing bottom from the " unsticking " point of view. For the type indicated above the coefficients would be : Midship area coefficient ... ... ... = 0-75 Coefficient of flneness-of waterplane ... = 0-68 Block coefficient ... ... ... ... = 0-47 It will be appreciated that by the tabulation of coefficients for particular types of seaplane, one can obtain more accurate figures for any particular type than those given above, but this is unnecessary, as will be indicated in the part of this article dealing with " Floats of Similar Form." COEFFICIENT y l\ H OF FINENESS — FIG.2 OF — WATERPLANE"68 _———""i Q The coefficients suggested here are the mean values of a large number of machines of the total weight range men- tioned, and can be utilised to give reasonable float propor- tions and lines. The method is similar to that used by naval architects ; the difference in application lies merely in the coefficient values used for seaplane practice. It is to be noted that the assumption is made that the seaplane has a considerable margin of power. MIDSHIP AREA COEFFICIENT = -75.A r /^~^ - FIG. 1. / D ! a c FIG. 3. \ \ FIG. 4.. \ The angle of entry must also be considered. This point will be dealt with more fully when outlining the problem of wave-interference. (d) The ratios of length, breadth and depth of the float are taken as follows :—> If L — overall length of float B = greatest breadth. D = greatest depth. Then D should equal 1 • 13 X B. And L „ 8 x B. In a racing machine the floats, however, would be of lesser beam to delay planing and the opposite would be the case for a machine whose margin of power was low. Assuming now a total float volume of 4,500 lbs. required, and using the coefficients and length breadth and depth ratios given, we get 0-47 x L x B x D = total vol. of float = 4,50064 cub. ft. 0-47 x 8B X B x 113B = 4,50064 cub. ft. 6l4d
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